ADSORPTION (a variation of the word °absorption"). The condensation of a gas or vapor upon the surface of a solid. The fact that solid bodies are capable of condensing upon their surfaces air films or gas films of. considerable density was probably first forced upon the attention of the physicist by the dif ficulty of obtaining a permanently good vacuum. Thus it was found that a glass globe (for example) might be highly ex hausted, and yet after a time the vacuum would be found to be materially reduced, even when it was apparently impossible that any air should have leaked in from without. It is now known that unless special pains are taken to prevent it, a film of air remains condensed against the surface of the glass, even when the vacuum through the general bulk of the globe is very high; and air molecules from this film are gradually given off until the vacuum becomes much less perfect than it was at first. To prevent this action it is customary to heat the vessel that is being exhausted, as the gas film is largely driven off from the walls of the vessel when they are heated. See VACUUM.
The condensation of gaseous films upon the surfaces of solids is undoubtedly due to the molecular attraction exerted by the upon the gas. This molecular attraction is in sensible at distances that are easily measurable, but it may be very great at points sufficiently near to the surface of the solid. The expres sion °sensible molecular attraction," which is in use among physicists, is indefinite, and no very precise statement can be made with regard to the limiting distance beyond which the at traction is not sensible; but from the investi gations of Quincke, Plateau, Maxwell, Kelvin and others, we may infer, in a general way, that molecular attraction is not sensible at a greater distance than about 1-200,000th of an inch. Hence it is safe to say that this is the
maximum thickness that the gas film condensed on a solid surface can have.
Concerning the condition of the gas in the film we can only say that where it is in im mediate contact with the solid it probably has a very great density, this density rapidly falling off as we pass away from the solid. Under ordinary conditions of temperature the air film condensed against a solid cannot actually be in the liquid state, because it is not possible for air to exist in this state at any temperature higher than 220° below zero F. See CRITICAL Poirrr.
It is well known that a solid body appears to weigh less when it has been recently heated, or is still hot, than it does when it has been al lowed to stand for some time in contact with the air at ordinary temperatures. This phenomenon is apparently due, to a consider able extent, to variations in the thickness of the film of air and moisture that the body con denses upon its surface. In accurate ther mometry (see THERMOMETER), where the as thermometer is used at a standard, great pains are taken, in filling the thermometer bulb with gas, to avoid the contamination of the ther mometric gas by moisture condensed upon the surface of the bulb; the bulb being repeatedly exhausted, heated and refilled, until there is no longer the smallest chance of any appreciable part of the original surface film remaining. The phenomena of adsorption have not yet been fully studied.